The broad aim of the proposed project is to experimentally separate the influences of energy in Na and Ca ionic electrochemical gradients, the activating effects of (Na) and (Ca), and the membrane potential on the Na:Ca counter-transport process in squid and Myxicola giant axons. A change in (Na)o, for example, can alter both the energy in the Na electrochemical gradient and the degree of activation of the Na:Ca carrier by (Na)o. This fact makes it difficult to investigate quantitatively the role of the Na electrochemical gradient which energizes Ca extrusion in excitable cells. This proposal seeks to circumvent this problem by investigating the effects of altering Na ionic concentrations at a constant value of the ratio (Na)o/(Na)i. This procedure does not change the Na equilibrium potential and, at constant membrane potential, does not change the energy in the electrochemical gradient for Na. By this method, activating and inhibitory effects of (Na)o and (Na)i on the Na:Ca carrier mechanism that extrudes Ca from the axon can be studied apart from energy considerations. The main methodology to be employed is that of internal dialysis which permits control of both inside and outside ionic concentrations by the investigator. In cases where the membrane potential must be held constant at different experimental values, a standard voltage clamp apparatus will be employed. A major objective via use of these techniques is to understand how Na and Ca ionic gradients and the membrane potential interplay in regulating the operation of the Na:Ca counter-transport mechanism in excitable cells.